If load is kept the same (gearing), then current will increase with an increase in voltage. If you regear to get the same top speed you have now, then yes, you'll actually end up with a little extra runtime (how much, I dunno!). Think of adding another A123 cell as like adding another 150-200 watt hours (I'm assuming 3.0 volts at 60 amps? I could be off with that number, so you may be adding slightly more or slightly less energy) to your battery pack. More energy in the pack with that extra cell, so if you regear so as to reduce your current draw back to what you were drawing originally with your current setup, if not less, then you will experience a runtime gain.